The present invention relates to graphics plotters and, more particularly, in a graphics plotter system having a platen over which paper is moved longitudinally to create one axis, penholding apparatus moved transversely across the platen and the paper to create the other axis, and a controlled raising and lowering mechanism connected to the penholding apparatus for lowering a pen being held therein into contact with and for raising it from contact with the paper and for providing a relative vertical position value signal, to the improvement for allowing automatic paper sizing comprising, vertical sensing means carried by the penholding apparatus for sensing the top surface of the platen; history means operably connected to the vertical sensing means and to receive the vertical position value signal from the controlled raising and lowering mechanism for calculating and saving values indicating the distance to the platen top surface at a plurality of points across the platen; edge sensing means carried by the penholding apparatus and cooperating with the vertical sensing means and the history means for sensing the edges of paper; means for automatically moving the paper from end to end over the platen and for saving the positional values of the ends of the paper as found by the edge sensing means; and, means for automatically moving the penholding apparatus from side to side over the paper and for saving the positional values of the side edges of the paper as found by the edge sensing means.
The original graphics plotters as employed with computers to create two-dimensional graphics and drawings comprised a flat table upon which the drawing paper was fastened and over which a printhead assembly was moved by a beam also moving on spaced tracks parallel to one another. Such plotters were large and cumbersome, particularly with larger so-called "D" and "E" sized drawings as typically used in commercial applications. More recently, a single, non-moving-beam system such as that represented by the critical elements thereof in FIG. 1 has been employed as the standard approach to plotting. In such a plotter, there is a platen 10 over which the paper (not shown) is draped to be moved longitudinally for one axis by a pair of driven pinch-rollers 12 positioned at the respective edges of the paper. The single beam 14 (typically in the form of a rod or bar) is disposed above and parallel to the top surface of the platen 10. A driven penholder 16 releasably gripping a pen 18 is moved transversely across the paper from side to side to create the other axis. An appropriate mechanism (not shown) is connected to raise and lower the pen 18 as by rotating the penholder 16 about the beam 14.
An important aspect of such plotters is the determination of paper size and position; that is, the plotter must "know" where the ends and side edges of the paper presently being used are located. Typically, the pinch-rollers 12 are movable from side to side to adjust for different paper widths. Thus, the paper can be positioned anywhere along the platen 10. As depicted in FIGS. 2 and 3, the 0,0 coordinate point of the drawing must be established and the paper size available for the drawing determined prior to the plotting procedure being begun. Thus, as shown in FIG. 2, the paper 24 must be moved from end to end over the platen 10 by the pinch-rollers 12 so that the internal logic of the plotter (not shown) can establish internal values necessary to position the writing tip 20 relative to the two ends of the paper 24 indicated by the arrows 26. In similar manner, as depicted in FIG. 3, internal values must be established with respect to the side edges of the paper 24 indicated by the arrows 28. Typically in the prior art, these functions are manually accomplished by a human operator employing a joystick, buttons, and the like. The paper 24 and penholder 16 are moved by the operator using the mechanical input devices until the writing tip 20 is at an end or side edge. A button, or the like, is depressed to indicate to the internal logic the present location of the writing tip 20, with reference to calibration. Presently, there is no economical and reliable way in which such a plotter can automatically self-determine the locations of the ends and side edges.
Wherefore, it is an object of the present invention to provide an economical and reliable method and apparatus whereby a plotter can automatically determine the postiions of the edges of paper therein and the size of that paper.
Other objects and benefits of the present invention will become apparent from the detailed description contained hereinafter taken in conjunction with the accompanying drawing figures.